SITE CONSIDERATIONS
For
EQUIPMENT INSTALLATION,
GROUNDING
&
WIRING
Bristol Babcock
Supplement Guide - S1400
Issue: 09/01
A Guide for the Protection of
Site Equipment & Personnel
In the Installation of
BBI Series 33XX DPCs & RTUs
NOTICE
Copyright Notice
The information in this document is subject to change without notice. Every effort has been
made to supply complete and accurate information. However, Bristol Babcock assumes no
responsibility for any errors that may appear in this document.
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and belong to their respective companies, or trademark holders.
Copyright (c) 2001 Bristol Babcock, 1100 Buckingham St., Watertown, CT 06795. No part of
this manual may be reproduced in any form without the express written permission of
Bristol Babcock.
Supplement S1400 Page 0-1 Table Of Contents
Supplement Guide S1400
SITE CONSIDERATIONS FOR EQUIPMENT
INSTALLATION, GROUNDING & WIRING
TABLE OF CONTENTS
SECTION TITLE PAGE #
Section 1 - INTRODUCTION
1.1 GENERAL INTRODUCTION .......................................................................................1-1
1.2 MAJOR TOPICS ............................................................................................................. 1-1
Section 2 - PROTECTION
2.1 PROTECTING INSTRUMENT SYSTEMS................................................................... 2-1
2.1.1 Quality Is Conformance To Requirements.................................................................... 2-1
2.2 PROTECTING EQUIPMENT & PERSONNEL ........................................................... 2-1
2.2.1 Considerations For The Protection of Personnel ..........................................................2-2
2.2.2 Considerations For The Protection of Equipment ........................................................2-2
2.3 OTHER SITE SAFETY CONSIDERATIONS............................................................... 2-3
Section 3 - GROUNDING & ISOLATION
3.1 POWER & GROUND SYSTEMS................................................................................... 3-1
3.2 IMPORTANCE OF GOOD GROUNDS......................................................................... 3-1
3.3 transients and interference............................................................................................ 3-2
3.4 EARTH GROUND CONNECTIONS.............................................................................3-2
3.4.1 Establishing a Good Earth Ground. .............................................................................. 3-2
3.4.1.1 Soil Conditions ................................................................................................................3-4
3.4.1.2 Soil Types ........................................................................................................................3-4
3.4.1.3 Dry, Sandy or Rocky Soil................................................................................................ 3-6
3.5 GENERAL RECOMMENDATIONS ............................................................................. 3-7
3.5.1 Noise and Signal Errors ................................................................................................. 3-7
3.5.2 General Considerations.................................................................................................. 3-8
3.5.3 Other Grounding Considerations. ................................................................................. 3-8
3.6 GROUNDING TECHNIQUES FOR SERIES 33XX SYSTEMS ................................3-12
3.6.1 Several DPCs Mounted in Metal Cabinet with Power Supply ..................................3-12
3.6.2 Multiple DPC Cabinets with Local Power Supply in Each Cabinet..........................3-14
3.6.3 Multiple DPC Cabinets Powered by Single Power Supply ........................................3-14
3.6.4 Multiple Clusters of DPC Cabinets Powered by Local Supplies................................ 3-16
Section 4 - LIGHTNING ARRESTERS & SURGE PROTECTORS
4.1 STROKES & STRIKES .................................................................................................. 4-1
4.1.1 Chance of Being Struck by Lightning. ..........................................................................4-1
4.1.2 Antenna Caution ............................................................................................................4-3
4.1.3 Ground Propagation ....................................................................................................... 4-5
4.1.4 Tying it all Together.......................................................................................................4-5
4.1.5 Impulse Protection Summary ........................................................................................ 4-5
4.2 USE OF LIGHTNING ARRESTERS & SURGE PROTECTORS................................ 4-6
Supplement S1400 Page 0-2 Table Of Contents
Supplement Guide S1400
SITE CONSIDERATIONS FOR EQUIPMENT
INSTALLATION, GROUNDING & WIRING
TABLE OF CONTENTS
SECTION TITLE ..................................................................................................... PAGE #
Section 5 - WIRING TECHNIQUES
5.1 OVERVIEW ....................................................................................................................5-1
5.2 INSTRUMENT WIRING. ..............................................................................................5-1
5.2.1 Common Returns............................................................................................................5-1
5.2.2 Use of Twisted Shielded Pair Wiring (with Overall Insulation).................................. 5-2
5.2.3 Grounding of Cable Shields. .......................................................................................... 5-3
5.2.4 Use of Known Good Earth Grounds .............................................................................. 5-3
5.2.5 Earth Ground Wires ....................................................................................................... 5-3
5.2.6 Working Neatly & Professionally .................................................................................. 5-3
5.2.7 High Power Conductors and Signal Wiring ..................................................................5-4
5.2.8 Use of Proper Wire Size .................................................................................................5-4
5.2.9 Lightning Arresters & Surge Protectors ....................................................................... 5-4
5.2.10 Secure Wiring Connections............................................................................................ 5-5
REFERENCE DOCUMENTS
1. IEEE Recommended Practice for Grounding of Industrial and Commercial Power Systems - ANSI/IEEE Std
142-1982
2. IEEE Guide for the Installation of Electrical Equipment to Minimize Electrical Noise inputs to Controllers
from External Sources - IEE Std 518-1982
3. Lightning Strike Protect; Roy B. Carpenter, Jr. & Mark N. Drabkin, Ph.D.; Lightning Eliminators &
Consultant, Inc., 6687 Arapahoe Road, Boulder Colorado
4. Lightning Protection Manual for Rural Electric Systems, NRECA Research Project 82-5, Washington DC,
1983
5. Grounding for the Control of EMI; Hugh W. Denny; Don White Consultants, Inc., 1983, 1
st
Edition
6. Fundamentals of EGM - Electrical Installations; Michael D. Price; NorAm Gas Transmission, 525 Milam
Street, Shreveport, Louisiana 71151
Section 1 - Overview Page 1-1 S1400
Section 1 - Overview
1.1 INTRODUCTION
This document provides information pertaining to the installation of BBI Series 33XX
systems; more specifically, information covering reasons, theory and techniques for
protecting your personnel and equipment from electrical damage. Your instrument system
affects the quality of service provided by your company and many aspects of its operational
safety. Loss of instruments means lost production and profits as well as increased expenses.
Information contained in this document is for educational purposes. Bristol Babcock makes
no warranties or guarantees on the effectiveness or the safety of techniques described herein.
Where the safety of installations and personnel is concerned, refer to the National Electrical
Code Rules and rules of local regulatory agencies.
1.2 MAJOR TOPICS
Topics are covered in seven sections designed to pinpoint major areas of concern for the
protection of site equipment and personnel. The following overview is provided for each of
the major sections.
· Section 2 - Protection
This section provides the reasons for protecting instrument systems. An overview of the
definition of quality and what we are trying to accomplish in the protection of site
installations and how to satisfy the defined requirements is presented. Additionally,
this section provides considerations for the protection of personnel and equipment.
· Section 3 - Grounding & Isolation
Information pertaining to what constitutes a good earth ground, how to test and
establish such grounds, as well as when and how to connect equipment to earth grounds
is provided
· Section 4 - Lightning Arresters & Surge Protectors
Some interesting information dealing with Lightning strikes and strokes is presented in
technical and statistical form along with a discussion of how to determine the likelyhood
of a lightning strike.
Protecting equipment and personnel during the installation of
radios and antenna is discussed in a review of the dangers to equipment and personnel
when working with antennas. Reasons for the use of lightning arresters and surge
protectors are presented along with overviews of how each device protects site
equipment.
· Section 5 - Wiring Techniques
Installation of Power and “Measurement & Control” wiring is discussed. Information on
obscure problems, circulating ground and power loops, bad relays, etc. is presented.
Good wire preparation and connection techniques along with problems to avoid are
discussed. This sections list the ten rules of instrument wiring.”
Section 2 - Protection Page 2-1 S1400
Section 2 - Protection
2.1 PROTECTING INSTRUMENT SYSTEMS
Electrical instrumentation is susceptible to damage from a variety of natural and man
made phenomena. In addition to wind, rain and fire, the most common types of system and
equipment damaging phenomena are lightning, power faults, communication surges &
noise and other electrical interference’s caused by devices such as radios, welders,
switching gear, automobiles, etc. Additionally there are problems induced by geophysical
electrical potential & noise plus things that are often beyond our wildest imagination.
2.1.1 Quality Is Conformance To Requirements
A quality instrumentation system is one that works reliably, safely and as purported by the
equipment manufacturer (and in some cases by the system integrator) as a result of good
equipment design and well defined and followed installation practices. If we except the
general definition of quality to be, “quality is conformance to requirements,” we must also
except the premise that a condition of “quality” can’t exist where requirements for such an
end have not been evolved. In other words, you can’t have quality unless you have
requirements that have been followed. By understanding the requirements for a safe, sound
and reliable instrumentation system, and by following good installation practices (as
associated with the personnel and equipment in question), the operational integrity of the
equipment and system will be enhanced.
Understanding what is required to properly install BBI equipment in various en-
vironments, safely, and in accordance with good grounding, isolating and equipment
protection practices goes a long way toward maintaining a system which is healthy to the
owner and customer alike. Properly installed equipment is easier to maintain and operate,
and is more efficient and as such more profitable to our customers. Following good in-
stallation practices will minimize injury, equipment failure and the customer frustrations
that accompany failing and poorly operating equipment (of even the finest design). Ad-
ditionally, personnel involved in the installation of a piece of equipment add to or subtract
from the reliability of a system by a degree which is commensurate with their technical
prowess, i.e., their understanding of the equipment, site conditions and the requirements
for a quality installation.
2.2 PROTECTING EQUIPMENT & PERSONNEL
Series 33XX installations must be performed in accordance with National Electrical Code
Rules, electrical rules set by local regulatory agencies, and depending on the customer
environment (gas, water, etc), other national, state and local agencies such as the American
Water Works Association (AWWA). Additionally, installation at various customer sites may
be performed in conjunction with a “safety manager” or utility personnel with HAZMAT
(hazardous material) training on materials present (or potentially present) as required by
OSHA, the customer, etc.
S1400 Page 2-2 Section 2 - Protection
2.2.1 Considerations For The Protection of Personnel
Always evaluate the site environment as if your life depended on it. Make sure that you
understand the physical nature of the location where you will be working. Table 2-1
provides a general guideline for evaluating an installation site.
Table 2-1 - Installation Site Safety Evaluation Guide
# Guide
1 Indoor or outdoor – Dress Appropriately
2 If outdoor, what kind of environment, terrain, etc. Watch out for local varmint (bees,
spiders, snakes, etc.)
3 If indoor or outdoor – determine if there are any pieces of dangerous equipment or any
processes which might be a risk to your safety
4 If in a tunnel, bunker, etc. watch out for a build up of toxic or flammable gases. Make
sure the air is good. Watch out for local varmint (bees, spiders, snakes, etc.)
5 Hazardous or Non-Hazardous Environment – Wear appropriate safety equipment and
perform all necessary safety measures.
6 Before installing any equipment or power or ground wiring, make sure that there are no
lethal (life threatening) voltages between the site where the instrument will be installed
and other equipment, pipes, cabinets, etc. or to earth itself.
7 Never assume that adjacent or peripheral equipment has been properly installed and
grounded. Determine if this equipment and the 33XX unit in question can be touched
simultaneously without hazard to personnel and/or equipment?
8 Before embarking to remote locations where there are few or no human inhabitants ask a
few simple questions like, should I bring water, food, hygienic materials, first aid kit, etc?
Be Prepared!
9 Observe the work habits of those around you – for your own safety!
Some of the items that a service person should consider before ever going on site can be
ascertained by simply asking questions of the appropriate individual. Obviously other
safety considerations can only be established at the installation site.
2.2.2 Considerations For The Protection of Equipment
Always evaluate the site installation/service environment and equipment. Understand the
various physical interfaces you will be dealing with such as equipment mounting and
supporting, analog and digital circuits, power circuits, communication circuits and various
electrical grounds. Table 2-2 provides a general guideline for evaluating the equipment
protection requirements of an installation site.
Table 2-2 - Equipment Protection Site Safety Evaluation Guide
# Guide Reference Section
1 Environment - Class I, Division 2 - Nonincendive
Environment - Class I, Division 1 - Intrinsically Safe
Other - Safe or unrated area
See Appendix A of CI Manual
See Appendix B of CI Manual
2 Earth Ground - Established by mechanical/electrical or
(both) or not at all.
See Section 3
3 Is the area prone to lightning strikes? See Section 4
4 Are there surge suppressors installed or to be installed? See Section 4
5 Are there overhead or underground power or com-
munication cables in the immediate area?
See Section 2.3
Section 2 - Protection Page 2-3 S1400
Table 2-2 - Equipment Protection Site Safety Evaluation Guide (Continued)
# Guide Reference Section
6 Is there an antenna in the immediate area? See Section 4.1.2
7 How close is other equipment? Can someone safely touch this
equipment and a 33XX unit simultaneously?
See Section 2.3
8 Determine equipment ground requirements. How will the 33XX unit
and its related wiring be grounded? Consider Earth Ground, Circuit
Ground, Conduit Ground, Site Grounds!
See Section 3
9 Are there any obviously faulty or questionable power or ground
circuits?
See Section 2.3
2.3 OTHER SITE SAFETY CONSIDERATIONS
Overhead or underground power or communication cables must be identified prior to
installing a new unit. Accidentally cutting, shorting or simply just contacting power,
ground, communication or process control I/O wiring can have potentially devastating
effects on site equipment, the process system and or personnel.
Don’t assume that it is safe to touch adjacent equipment, machinery, pipes, cabinets or even
the earth itself. Adjacent equipment may not have been properly wired or grounded, may be
defective or may have one or more loose system grounds. Measure between the case of a
questionable piece of equipment and its earth ground for voltage. If a voltage is present,
something is wrong.
AC powered equipment with a conductive case should have the case grounded. If you don’t
see a chassis ground wire, don’t assume that it is safe to touch this equipment. If you notice
that equipment has been grounded to pipes, conduit, structural steel, etc., you should be
leery. Note: AWWA’s policy on grounding of electric circuits on water pipes states,
“The American Water Works Association (AWWA) opposes the grounding of
electrical systems to pipe systems conveying water to the customer’s premises….”
Be sure that the voltage between any two points in the instrument system is less than the
stand-off voltage. Exceeding the stand-off voltage will cause damage to the instrument and
will cause the instrument to fail.
Section 3 - Grounding & Isolation Page 3-1 S1400
Section 3 - Grounding & Isolation
3.1 POWER & GROUND SYSTEMS
Series 33XX DPCs and RTUs (3305, 3310, 3330, 3331, 3332, 3335) utilize DC power
systems. With the exception of some 3305 SAPs, AC power supplies are not provided with
these units. Series 3305 SAP RTUs and 3335 DPCs are provided with a Ground Lug that
accommodates up to a #4 AWG size wire for establishing a connection to Earth Ground.
Table 3-1 provides ground connection information for these units.
Model Gnd. Jumpers Doc. Ref. Location Notes
3305 RTU None CI-3305 - C2 MI/OB Bd.
# 12 AWG Wire
Pin 1 = PCOM (DC Ret.)
Pin 2 = POWER (DC+)
Pin 3 = CHASSIS
3305 SAP None
CI-3305 - Sup 1
Section 2.2.3
Ground Lug
#4 AWG - see Figs.
S1-5, -6
3310 RTU
W1A, W1B
See C3A - topic
Grounding Option
CI-3310 - C3A MFIB Bd.
#14 AWG Wire - see Figs.
3A-1, -2, -4, -6
3330 DPC
3332 RED
W1A, W1B, W1C
See C3A - topic
Grounding Option
CI-3330 - C3A
CI-3330 - C3K
SI Bd.
#14 AWG Wire - see Figs.
3A-1, -2, -3, 3K-7, 3K-8
3331 RIO
3335 DPC
W13A, W13B
See C3A - topic
Grounding Option
CI-3335 - C3L
CI-3335 - C3A
Jumpers = SI Bd.
Wiring = System
Monitor Module
& Ground Lug
# 12 AWG Wire from
SMM - TB1 pin 2 (GND)
to Zero Ref. Point - see
Figs 3A-1, -2, -7, -8, -9
#4 AWG Wire from
Chassis Bonding Bolt to
Power Grid Ground
3.2 IMPORTANCE OF GOOD GROUNDS
Series 33XX DPCs and RTUs are utilized in instrument and control systems that must
operate continually and within their stated accuracy over long periods of time with
minimum attention. Failures resulting from an improperly grounded system can become
costly in terms of lost time and disrupted processes. A properly grounded system will help
prevent electrical shock hazards resulting from contact with live metal surfaces, provide
additional protection of equipment from lightning strikes and power surges, minimize the
effects of electrical noise and power transients, and reduce signal errors caused by ground
wiring loops. Conversely, an improperly grounded system may exhibit a host of problems
that appear to have no relationship to grounding. It is essential that the reader (service
technician) have a good under-standing of this subject to prevent needless troubleshooting
procedures.
WARNING
This device must be installed in accordance with the National
Electrical Code (NEC) ANSI/NEPA-70. Installation in hazardous
locations must also comply with Article 500 of the code.
S1400 Page 3-2 Section 3 - Grounding & Isolation
3.3 TRANSIENTS AND INTERFERENCE
The extensive use of low-power integrated circuitry in modern electronic equipment
requires proper grounding techniques to insure reliable system operation. The following
checklist will help identify some critical areas:
1. All instrumentation devices at the site should be checked so that no potential greater
than the standoff voltage can exist within or between devices.
2. To minimize outside signal interference and provide equipment protection from
lightning or transients, the earth ground at the site must be tested to insure that its
impedance measures less than 10 ohms at 7 MHz. This qualification is essential since a
transient potential or an interference signal at the instrument site can vary over the
entire electromagnetic spectrum from DC to several hundred MHz.
Note that transients can be produced through natural phenomena and man-made
conditions. Natural transients may result from lightning (7-14 MHz), static (many
frequencies), and wind (DC charge and static). Man-made transients can result
from defective light bulbs or electrical appliances, sudden electrical load shifts,
inductive load surges, arcing contacts and poor AC power connections.
3. If radio frequency (RF) interference is present at the input of an instrument, observe if
it has a consistent or irregular pattern. Constant interference can come from
commercial radio stations, while irregular interference
can come from private stations.
Although shielding and grounding will eliminate or minimize most cases of RF
interference, obstinate cases may require attenuation filters.
RF interference can also be caused by power companies that apply modulated RF
signals to power lines to communicate data. Other RF noise sources include digital
clocks, computers, relay contacts, motors transformers, switches, arc welders, etc.
3.4 EARTH GROUND CONNECTIONS
To properly ground a DPC or RTU unit, the units Chassis Ground (post or terminal) must
ultimately be connected to a known good Earth Ground. Observe recommendations
provided in topics Establishing a Good Earth Ground and Ground Wire Considerations
.
3.4.1 Establishing a Good Earth Ground
A common misconception of a ground is that it consists of nothing more than a metal pipe
driven into the soil. While such a ground may function for some applications, it will often
not be suitable for a complex system of sophisticated electronic equipment. Conditions such
as soil type, composition and moisture will all have a bearing on ground reliability.
A basic ground consists of a 3/4-inch diameter rod with a minimum 8-foot length driven into
conductive earth to a depth of about 7-feet as shown in Figure 3-1. Number 3 or 4 AWG
solid copper wire should be used for the ground wire. The end of the wire should be clean,
free of any coating and fastened to the rod with a clamp. This ground connection should be
covered or coated to protect it from the weather and the environment.
Section 3 - Grounding & Isolation Page 3-3 S1400
Figure 3-1 - Basic Ground Rod Installation
Figure 3-2 - Overhead Map of Ground Bed for Gas Metering Station
S1400 Page 3-4 Section 3 - Grounding & Isolation
3.4.1.1 Soil Conditions
Before installing a ground rod, the soil type and moisture content should be analyzed.
Ideally, the soil should be moist and moderately packed throughout to the depth of the
ground rod. However, some soils will exhibit less than ideal conditions and will require
extra attention.
Soil types can be placed into two general categories with respect to establishing and
maintaining a good earth ground, i.e., ‘Good Soil’ and ‘Poor Soil.’
To be a good conductor, soil must contain some moisture and free ions (from salts in the
soil). In very rainy areas, the salts may be washed out of the soil. In very sandy or arid area
the soil may be to dry and/or salt free to a good conductor. If salt is lacking add rock salt
(NaCl); if the soil is dry add calcium chloride (CaCl
2
).
3.4.1.2 Soil Types: Good Poor
Damp Loam Back Fill
Salty Soil or Sand Dry Soil
Farm Land Sand Washed by a Lot of Rain
Dry Sand (Desert)
Rocky Soil
Ground Beds must always be tested for conductivity prior to being placed into service. A
brief description of ground bed testing in ‘Good Soil’ and ‘Poor Soil’ is provided herein.
Details on this test are described in the National Electrical Code Handbook. Once a reliable
ground has been established, it should be tested on a regular basis to preserve system
integrity.
Figure 3-3 - Basic Ground Bed Soil Test Setup
Section 3 - Grounding & Isolation Page 3-5 S1400
Figure 3-3 shows the test setup for ‘Good Soil’ conditions. If the Megger* reads less than 5
ohms, the ground is good. The lower the resistance, the better the earth ground. If the
Megger reads more than 10 ohms, the ground is considered ‘poor.’ If a poor ground is
indicated, one or more additional ground rods connected 10 feet from the main ground rod
should be driven into the soil and interconnected via bare AWG 0000 copper wire and 1” x
¼-20 cable clamps as illustrated in Figure 3-4). * Note: Megger is a Trademark of the
Biddle Instrument Co. (now owned by AVO International). Other devices that
may be used to test ground resistance are “Viboground”; Associated Research,
Inc., “Groundmeter”; Industrial Instruments, Inc., and “Ground-ohmer”; Herman
H. Sticht Co., Inc.
If the Megger still reads more than 10 ohms, mix a generous amount of cooking salt, ice
cream salt or rock salt with water and then pour about 2.5 to 5 gallons of this solution
around each rod (including the test rods). Wait 15 minutes and re-test the soil. If the test
fails, the soil is poor and a ‘Poor Soil Ground Bed’ will have to be constructed.
Figure 3-4 - Basic Ground Bed Soil Test Setup with Additional Ground Rods
Figure 3-5 shows a typical Poor Soil Ground Bed Electrode. A Poor Soil Ground Bed will
typically consists of four or more 10-foot long electrodes stacked vertically and separated by
earth. Figure 3-6 shows the construction of a Poor Soil Ground Bed. For some poor soil
sites, the ground bed will be constructed of many layers of ‘Capacitive Couplings’ as
illustrated. In extremely poor soil sites one or more 3’ by 3’ copper plates (12 gauge or 1/16”
thick) will have to be buried in place of the electrodes.
Figure 3-5 - Ground Electrode Construction for Poor Soil Conditions
S1400 Page 3-6 Section 3 - Grounding & Isolation
3.4.1.3 Dry, Sandy or Rocky Soil
Very dry soil will not provide enough free ions for good conductance and a single ground rod
will not be effective. A buried counterpoise or copper screen is recommended for these
situations. It will be necessary to keep the soil moist through regular applications of water.
Sandy soil, either wet or dry, may have had its soluble salts leached out by rain water,
thereby reducing conductivity of the ground. High currents from lightning strikes could also
melt sand and cause glass to form around the ground rod, rendering it ineffective. A buried
counterpoise or copper screen is preferred for these installations along with regular
applications of salt water.
Rocky soil can pose many grounding problems. A counterpoise or copper plate will probably
be required. Constructing a trench at the grounding site and mixing the fill with a
hygroscopic salt such as calcium chloride may help for a time. Soaking the trench with
water on a regular basis will maintain conductivity.
Figure 3-6 - Poor Soil Ground Bed Construction Diagram
Section 3 - Grounding & Isolation Page 3-7 S1400
3.5 GENERAL RECOMMENDATIONS
When wiring equipment into a system, the electrical conduit must have a diameter that will
accommodate the desired number of wires. The cross- sectional area of the conduit should
be large enough to allow the wires to be pulled through without excessive tightness or
binding. A conduit that is too tight can shred insulation, damage wiring, and result in
possible opens, shorts, or intermittent effects. Such conditions are often difficult to trace
because the defect is concealed inside the conduit.
3.5.1 Noise and Signal Errors
Noise and signal errors are often the result of poor wiring and grounding practices. Some
common problem areas are listed as follows:
o Shielding AIs and AOs. Very often analog DC signal leads must run parallel to
wires radiating AC fields, pulse information, or switching transients. Due to
inductive and capacitive pickup, some of this information can leak into an analog I/O
and cause peculiar effects in the control systems. To minimize or eliminate this
problem, the use of insulated and shielded, twisted lead pairs is recommended
between the external devices (transmitters, sensors, etc.) and the instrument inputs
(controllers, recorders, etc.).
The shields of each analog signal source should only be grounded at the input of the
in-strument. In some equipment, the shield will connect to the instrument chassis.
In other equipment, a "shield" terminal will be provided with several grounding
options. The user should refer to the instrument manual and follow grounding
recom-mendations.
o Common Returns. The use of a single "common" return wire for two or more input
signals is not recommended. This approach may introduce system ground loops that
cause erroneous readings at the instrument. Shielded transmitter or sensor wires
should be grounded at the input of the instrument, or connected to a shield terminal
(where provided) to prevent "sneak" ground paths.
o Discrete Outputs. Instruments provided with bi-state discrete outputs perform
functions such as control switching, alarm switching or pulse duration com-
munications. These outputs are furnished as either open collector or relay contact
outputs that operate at low power levels. While these levels are sufficient to operate
many devices, some will require much higher power levels. The use of external
amplifiers or repeating relays to drive end devices will prevent output overload and
add to the reliability of the system.
o Placement of Wiring. The dressing or physical placement of wiring requires close
scrutiny. Cables inside cabinets should be neatly secured at regular intervals.
Cables running between cabinets at different locations should be placed in conduits.
The cable length should allow sufficient slack for routine operational checks and
maintenance of the equipment. Wiring from input signal circuits and power circuits
should be separated as much as possible to minimize noise and transient pickup.
Power and signal leads should be run in separate conduit to minimize inductive
pickup.
S1400 Page 3-8 Section 3 - Grounding & Isolation
o Terminal Lugs. The use of crimp-type terminal lugs as connections for screw
terminals should be avoided. Terminal lugs, in many industrial climates, can be
affected by hidden corrosion. It is preferable to tin the wire end with solder and loop
it around the terminal screw. The screw should be tightened sufficiently to hold the
lead in place but not excessively so that the lead is sheared or the screw is stripped.
Equipment furnished with compression-type terminals includes an opening for
inserting tinned ends.
3.5.2 General Considerations
The following considerations are provided for the installation of system grounds:
i Size of ground wire (running to Earth Ground should be #4 AWG. It is recommended
that stranded copper wire is used for this application and that the length should be as
short as possible.
i This ground wire should be clamped or brazed to the Ground Bed Conductor (that is
typically a stranded copper AWG 0000 cable installed vertically or horizontally).
i The wire ends should be tinned with solder prior to installation.
i The ground wire should be run such that any routing bend in the cable has a
minimum radius of 12-inches below ground and 8-inches above ground.
The units Earth Ground Cable should be clamped to an exposed Ground Rod or to an AWG
0000 stranded copper Ground Cable that in turn should be connected to either an Earth
Ground Rod or Earth Ground Bed. Both ends of the units Earth Ground Cable must be free
of any coating such as paint or insulated covering as well as any oxidation. The connecting
point of the Ground Rod or AWG 0000 Ground Cable must also be free of any coating and
free of oxidation. Once the ground connection has been established (at either the Ground
Rod or Ground Cable) it should be covered or coated to protect it from the environment.
3.5.3 Other Grounding Considerations
Instrument enclosures, measuring devices, metal process vats, metal piping, and other
associated mechanical and electrical devices should all be grounded. The method of
grounding an instrument rack is shown in Figure 3-6. In this application the ground lead
typically attaches to a ground bus that is common to all equipment in the rack.
For applications employing equipment that communicates over telephone lines, a lightning
arrester Must Be provided. For indoor equipment the lightning arrester must be installed
at the point where the communication line enters the building as shown in Figure 3-7. The
ground terminal of this arrester must connect to a ground rod and/or a buried ground bed.
Applications that use transmitters or transducers require grounding and shielding. In
Figure 3-8, the ground conductor feeds through the electrical conduit and connects to the
ground screw of the transmitter even though the support pipe is grounded. However, if the
transmitter uses shielded wiring for its signal output, the shield should not be grounded at
the transmitter. For maximum signal accuracy, the shield should only be grounded at one
point in the system, typically at the input of the associated equipment.
Section 3 - Grounding & Isolation Page 3-9 S1400
Figure 3-6 - Grounding of Equipment Housing
Figure 3-7 - Grounding of Phone Line
S1400 Page 3-10 Section 3 - Grounding & Isolation
Figure 3-8 - Grounding of Transmitter
Figure 3-9 - Grounding of Thermometer Well in Gas Line
Section 3 - Grounding & Isolation Page 3-11 S1400
Gas lines also require special grounding considerations. If a gas meter run includes a
thermocouple or RTD sensor installed in a thermowell, the well (not the sensor) must be
connected to a gas discharge-type lightning arrestor as shown in Figure 3-9. A copper braid,
brazed to the thermal well, is dressed into a smooth curve and connected to the arrestor as
shown. The curve is necessary to minimize arcing caused by lightning strikes or high static
surges. The path from the lightning arrestor to the ground bed should also be smooth and
free from sharp bends for the same reason.
The ac power required to operate a system typically includes a service transformer located
at the street and a main breaker box and rate meter assembly at the building as shown in
Figure 3-10. The service transformer is grounded at the company's pole, while the breaker
box is grounded at the building. A lightning arrestor should be included at the breaker box
in each phase of the AC line, and each arrestor should be grounded accordingly.
Figure 3-10 - AC Power Grounding System
S1400 Page 3-12 Section 3 - Grounding & Isolation
3.6 GROUNDING TECHNIQUES FOR SERIES 33XX SYSTEMS
When installing a system that includes a number of Bristol Babcock, Series 33XX
Distributed Process Controllers (DPCs), it is essential to follow the procedures set forth by
the National Electrical Code (NEC) to minimize risk of equipment damage and electrical
shock.
WARNING
Electrically powered equipment must be properly grounded to
protect users from electrical shock and injury. All such devices
must be installed, wired, and grounded in accordance with the
National Electrical Code (NEC).
Series 33XX DPCs employ a power grid ground terminal (CHASSIS) and an instrument
ground terminal (24VRET) that connects to the "zero reference point" of the system.
Improper grounding of these terminals can produce multiple ground paths throughout the
system and result in increased noise pickup and signal offset errors. If more information is
required on this subject, the reader should refer to the publications cited at the end of this
document.
The examples that follow describe the grounding techniques for several types of Bristol
Babcock systems employing DPCs. Refer to the system description that is closest to your
application.
3.6.1 Several DPCs Mounted in Metal Cabinet with Power Supply
A small system can consist of one or more DPCs mounted in a single metal cabinet or rack
with a power supply. A power wiring diagram for this arrangement is shown in the example
of Figure 8. The following installation procedures apply:
1. Instrument Ground. The instrument ground of the DPCs (24VRET terminal of each
DPC) must connect to a terminal block within the cabinet that is electrically isolated
from the cabinet frame. This terminal block must provide termination for all in-
strument grounds and include termination for a multistranded, insulated, #4 gauge
wire (or greater). This wire, which will connect to the "zero reference point" of the
facility, must be run through metal conduit (pipe). Only the #4 wire will be contained
in this conduit. The conduit must also be connected by bonding strap to the cabinet
and facility frame as described in the NEC.
2. Setting DPC Power Jumpers. If the DPC is a Model 3335 or 3310, jumpers W1A and
W1B on the System Interconnect Board must be removed to isolate the chassis
connection from the 24VRET connection (see Figure 3-11). If it is a Model 3330,
jumpers W1A, W1B and W1C on the System Interconnect Board must be removed.
Series 3308 Gas Flow Computers or Correctors, if used with these systems, provide an
isolated instrument ground without setting jumpers.
3. AC Power Source. The 24 Vdc power supply requires a 120 Vac power source. The ac
power terminals of this supply are identified in Figure 3-11. The 120 Vac wiring for
this supply must be contained in cable trays along with the power grid grounding
wire. Figure 3-12 illustrates the cable tray layout and grounding points of a typical
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